Studies on radiation hardness of DEPFET-like test structures

The Vertex detector of the recently approved Belle II experiment will be equipped with DEPFET pixel sensors in its two innermost layers. Due to the increased luminosity which is expected at the collision point of the SuperKEKB accelerator, the detector suffers from ionizing damage of the insulating gate oxide. By ionizing radiation electron-hole pairs are generated in the oxide and positive charge is trapped and accumulated, resulting in a shift of the threshold voltage. This shift has to be corrected during the life-time of the experiment and the steering chips on the modules will have to cover the new voltage range. One possibility to reduce the resulting voltage shifts of the transistors is to use thinner gate dielectrics. In the laboratory this kind of defect in the gate oxide is investigated by the use of an x-ray tube at KIT. Irradiations on different DUTs have been done and results will be presented.